San Francisco AQI jumps to 271 on Friday, worst air quality ever recorded in the city

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Smothered in a thick, sooty blanket of toxic smoke, San Francisco's air quality index jumped to a jaw-dropping 271 Friday afternoon.

This hourly reading at 2 p.m. falls with within the "very unhealthy" range, meaning outdoor activity should be limited.

Across the Golden Gate in San Rafael, the air was also crummy with a reading of 209 in the "very unhealthy" range. In Oakland, the reading was nearly as bad as San Francisco's at 256.

"These are the highest levels we've had in the Bay Area ever since I've been here," says Richard Muller, a physics professor at UC Berkeley who co-founded the Berkeley Earth site tracking air quality around the world.

The acrid smoke from the Camp Fire irritates the lungs and stings the eyes, and Northern California residents from Butte County to the Bay Area are staying inside, huddled next to air filters, to limit smoke exposure. Many school districts canceled class Friday and San Francisco Airport has reported delays due to limited visibility all week.

On Thursday, the index that measures the public health impacts of pollution, peaked in San Francisco at 245, breaking the record-hitting hourly readings the city saw in October 2017 when the North Bay wildfires were burning. The Sacramento Valley, Chico area and Livermore have also seen "hazardous" levels in recent days.

The current weather pattern with a light offshore breeze from the Sacramento Valley blowing directly into the Bay Area is a perfect recipe for delivering smoke into the region. The smoke is pouring into the San Francisco area and it's moving slowly enough that it can't make its way over coastal mountains. The only way out is the Golden Gate and that's a narrow opening.

What's more, S.F.'s natural air-conditioner — the ocean breeze — has been turned off with a high-pressure system over the area and blocking Pacific systems. San Franciscans are used to breathing that fresh ocean air but this week they've received little of it.

"There will be a change in wind direction, and that clean marine air coming in from the Pacific," says Spencer Tangen, a meteorologist with the National Weather Service's Monterey office. " That onshore wind will be stronger, and it'll help to mix out the smoke, and push the smoke out."

There's also a 30 to 40 percent chance of rain on Wednesday, and Tangen says, "The rain will actually remove some of the smoke itself." Rain also returns to the forecast on Thursday, with a 40 to 50 percent chance.